Many sugars occur in a chain as well as in a ring structure and in solution it is possible to have an equilibrium between these two forms.

The aldehyde can be oxidized via a redox reaction. The chemical that causes this oxidation becomes reduced. Thus, a reducing sugar is one that reduces certain chemicals. Even though a ketone cannot be oxidized directly, a keto sugar may be converted to an aldehyde via a series of tautomeric shifts to migrate the carbonyl to the end of the chain. Therefore, keto sugars are also reducing.

Sugars having acetal or ketal linkages are not reducing sugars, as they do not have free aldehyde chains. They therefore do not react with any of the reducing-sugar test solutions. However, a non-reducing sugar can be hydrolysed using dilute hydrochloric acid. After hydrolysis and neutralization of the acid, the product may be a reducing sugar that gives normal reactions with the test solutions.

A sugar alcohol (also known as a polyol, polyhydric alcohol, or polyalcohol) is ahydrogenated form of carbohydrate, whose carbonyl group (aldehyde or ketone, reducing sugar) has been reduced to a primary or secondary hydroxyl group (hence the alcohol). Sugar alcohols have the general formula H(HCHO)n+1H, whereas sugars have H(HCHO)nHCO. In commercial foodstuffs sugar alcohols are commonly used in place of table sugar (sucrose), often in combination with high intensity artificial sweeteners to counter the low sweetness. Of these, xylitol is perhaps the most popular due to its similarity to sucrose in appearance and sweetness. Sugar alcohols do not contribute to tooth decay.

The simplest sugar alcohol, ethylene glycol, is the sweet, but notoriously toxic, chemical used in antifreeze. The higher sugar alcohols are for the most part nontoxic.

Sugar alcohols as food additives

As a group, sugar alcohols are not as sweet as sucrose, and they have less food energy than sucrose. Their flavor is like sucrose, and they can be used to mask the unpleasant aftertastes of some high intensity sweeteners. Sugar alcohols are not metabolized by oral bacteria, and so they do not contribute to tooth decay. They do not brown or caramelize when heated.

Sugar alcohols are usually incompletely absorbed into the blood stream from the small intestines which generally results in a smaller change in blood glucose than "regular" sugar (sucrose). This property makes them popular sweeteners among diabetics and people on low-carbohydrate diets. However, like many other incompletely digestible substances, overconsumption of sugar alcohols can lead to bloating, diarrhea and flatulence because they are not absorbed in the small intestine. Some individuals experience such symptoms even in a single-serving quantity. With continued use, most people develop a degree of tolerance to sugar alcohols and no longer experience these symptoms. As an exception, erythritol is actually absorbed in the small intestine and excreted unchanged through urine, so it has no side effects at typical levels of consumption.

The table to the right presents the relative sweetness and food energy of the most widely-used sugar alcohols. Despite the variance in food energy content of sugar alcohols, EU labeling requirements assign a blanket value of 2.4 kcal/g to all sugar alcohols.

Question:A bit confused because both the clinistix method and the benidicts test are ment to test for glucose, yet they both come up as negitive??

Answers:What is notable about sucrose is that unlike most disaccharides, the glycosidic bond is formed between the reducing ends of both glucose and fructose, and not between the reducing end of one and the non reducing end of the other. This linkage inhibits further bonding to other saccharides. Since it contains no anomeric hydroxyl groups, it is classified as a non reducing sugar.
Sucrose is non-reducing in Benedict's Reagents. Therefore, no colored precipitate is formed.

Answers:Reducing capability is defined by the presence of free or potential aldehyde or ketone group.
All monosaccharides have free ketone or aldehyde group. this means that they are all reducing sugars.
Maltose and sucrose are disaccharides, which means that they are made up of two monosaccharides.
Maltose is made up of two glucose units while sucrose is made up of glucose and fructose. The reducing ability of disaccharides is defined by the presence of a potential aldehyde or ketone group.
In the ring structures of sucrose and maltose, you have an anomeric carbon. this is the carbon which was hydrolyzed in the straight-chain structure. This is also the carbon that can open up the ring structure and reduce a metal ion.
Maltose's anomeric carbon is "free" and can therefore open up the ring and reduce the metal ion.
On the other hand, sucrose's anomeric carbon is not "free" since this carbon is used to link fructose and glucose together. therefore, this anomeric carbon can't open up the ring structure and react with the reagent.

Question:Why is Sucrose a non-reducing sugar when it has two anomeric carbons and only one
glycosidic linkage?

Answers:The two sugar rings are linked at their anomeric carbons, and so neither one has a free hemiacetal. In other words, sucrose is a glycoside that ties up both anomeric carbons as full acetals, and neither sugar is in equilibrium with its open-chain aldehyde form.

Answers:Reducing Sugars :
Sugars that contain aldehyde groups that are oxidised to carboxylic acids are classified as reducing sugars.
Common test reagents are :
Benedicts reagent (CuSO4 / citrate)
Fehlings reagent (CuSO4 / tartrate)
They are classified as reducing sugars since they reduce the Cu2+ to Cu+ which forms as a red precipitate, copper (I) oxide.
Remember that aldehydes (and hence aldoses) are readily oxidised (review ?)
In order for oxidation to occur, the cyclic form must first ring-open to give the reactive aldehyde.
So any sugar that contains a hemi-acetal will be a reducing sugar.
But glycosides which are acetals are not reducing sugars.
A reducing sugar is any sugar that, in basic solution, forms some aldehyde or ketone. This allows the sugar to act as a reducing agent, for example in the Maillard reaction and Benedict's reaction. Reducing sugars include glucose, glyceraldehyde, lactose, arabinose and maltose. All monosaccharides which contain ketone groups are known as ketoses, and those which contain aldehyde groups are known as aldoses. Significantly, sucrose is not a reducing sugar. It is in fact known as a non-reducing sugar.
Benedict's reagent is used to determine if a reducing sugar is present. If it is a reducing sugar, the mixture will turn green/orange/red. Fehling's solution can also be used for the same purpose, as both contain copper (II) ions, which are reduced to a brick red precipitate of copper (I) oxide when the solution is heated.
A reducing sugar occurs when its anomeric carbon is free. Since sugars occur in a chain as well as a ring structure, it is possible to have an equilibrium between these two forms. When the hemi-acetal or ketal hydroxylgroup is free, it is not locked, not linked to another (sugar)molecule, the aldehyde (or keto-) form (i.e. the chain-form) is available for reducing copper (II) ions. When a sugar is oxidized its carbonyl group (i.e. aldehyde or ketone group) is converted to a carboxyl group.
Non REDUCING SUGARS :
Fructose, glucose and galactose are all hexoses. However, whereas glucose and galactose are aldoses (reducing sugars), fructose is a ketose (a non-reducing sugar). It also has a five-atom ring rather than a six-atom ring. Fructose reacts with glucose to make the dissacharide sucrose.
the chemical difference between reducing and non-reducing sugars
Reducing sugars (monosaccharides, e.g. glucose and some disaccharides, e.g. lactose and maltose) act as reductants when heated with a weakly alkaline solution of copper (II) sulfate (e.g. Benedict s solution) to form an orange-brown precipitate of copper (I) oxide.
The Cu2+ is reduced to Cu+, which reacts with OH- to give Cu2O.
2Cu2+(aq) + 2e- + H2O(l) Cu2O(s) + 2H+(aq)
Aldehyde (-CHO) group in a reducing sugar can be the source of electrons that reduces copper (II) to copper (I)
R-CHO (aq) + H2O(l) R-COOH(aq) + 2H+(aq) + 2e-
Benedict test of glucose and sucrose, University of Manitoba, Winnipeg, Manitoba, Canada.
Fehling s solution (strongly alkaline copper sulfate) and Tollen s reagent (silver nitrate) can also be used.
Negative (1) and positive(2) Fehling test , Sciences de la vie et de la terre, Didier,(Sciences of life and the earth) Paris, France.
The reaction only occurs if the carbohydrate exists in equilibrium between a ring and open-chain form. The open-chain form contains CHO and CO-CH2OH functional groups, which are easily oxidised to carboxylic acids R-COOH and R-CHOH-COOH respectively. Since the carbohydrates are oxidised the reagent must have been reduced, therefore the carbohydrates are reducing agents. Thus the sugars that cause a reaction are called reducing sugars.

From Youtube

Reducing and Non-reducing Sugars :This webcast takes a look at notation for disaccharides and the difference between reducing and non-reducing sugars.